Premium medical diagnostic ultrasound imaging systems require a comprehensive set of imaging modes. These are the major imaging modes used in clinical diagnosis and include timeline Doppler, color flow Doppler, B mode and M mode. In the B mode, such ultrasound imaging systems create two-dimensional images of tissue in which the brightness of a pixel is based on the intensity of the echo return. Alternatively, in a color flow imaging mode, the movement of fluid (e.g., blood) or tissue can be imaged. Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The phase shift of backscattered ultrasound waves may be used to measure the velocity of the backscatterers from tissue or blood. The Doppler shift may be displayed using different colors to represent speed and direction of flow. In the spectral Doppler imaging mode, the power spectrum of these Doppler frequency shifts are computed for visual display as velocity-time waveforms.
One of the primary advantages of Doppler ultrasound is that it can provide noninvasive and quantitative measurements of blood flow in vessels. Given the angle between the insonifying beam and the flow axis (hereinafter referred to as the "Doppler angle"), the magnitude of the velocity vector can be determined by the standard Doppler equation: EQU v=cf.sub.d /(2f.sub.0 cos .theta.) (1)
where c is the speed of sound in blood, f.sub.0 is the transmit frequency and f.sub.d is the motion-induced Doppler frequency shift in the backscattered ultrasound signal.
In conventional ultrasound spectral Doppler imaging, the operator is required to manually position the sample gate to the measurement location in a two-dimensional image with or without color flow data. The operator also needs to manually adjust the sample gate size relative to the diameter of the vessel to be studied. From the acoustic data acquired over many transmit firings, Doppler frequency spectral data is obtained via standard Fast Fourier Transform (FFT) spectral analysis.
For a given measured Doppler frequency shift f.sub.d, the flow velocity (speed) v is calculated using Eq. (1). The ideal Doppler angle is zero, i.e., when the beam is aligned in the direction of blood flow. Unfortunately, Doppler angles that can be formed in practice tend to be larger, and as they approach 90 degrees, a small error in the angle estimate can lead to a large error in v. For this reason, it is generally recommended that the Doppler beam be steered to form Doppler angles of no greater than about 60 degrees for reliable velocity measurements. Angle steering is another adjustment the operator needs to make manually in conventional Doppler systems.
In an attempt to minimize manual Doppler adjustments when the spectral Doppler mode is activated, conventional scanners generally provide presets for the Doppler sample gate position and size, and for the beam steering angle. However, such presets have limited benefits because the vessel depth, size and orientation relative to the probe can vary a great deal from one case study to the next.
U.S. Pat. No. 5,365,929 describes the use of multiple range gates and multiple Doppler beams to scan a region of interest. By comparing some signal characteristic, such as total power or maximum velocity, of the multiple sample volumes, the scanner automatically selects the best sample gate for full spectral analysis and display. It will appear to the user that the scanner has automatically positioned the sample gate at a location where the Doppler signal is optimal in some sense.
European Patent Application No. 0 842 638 A2 describes a method of tracking vessel walls in the B-mode image, and then automatically adjusting the sample volume size to ensure the entire vessel diameter is covered. While this may be useful for volume flow measurements, the user is still expected to first manually position the sample volume and vessel wall markers at the correct locations. Also, in Doppler exams that do not involve volume flow measurements, different clinics may follow different practices in terms of sample gate size relative to the vessel diameter.
European Patent Application No. 0 985 380 A1 describes a method for automatic positioning of the Doppler sample gate based on bloodstream or color flow information. Among various specific applications, this method can be used to automatically set the sample gate cursor at an optimal position when the sample gate is first brought up in the image, or when it is being moved. The optimal position may be defined by a color flow pixel showing the highest velocity, or the center point of the largest flow segment, or the center point of the next best flow segment etc.
U.S. Pat. No. 5,690,116 describes a method of estimating the orientation (slope) of the vessel axis based on gray-scale image data, and then computing the Doppler angle.
U.S. Pat. No. 6,068,598 describes a robust method for detecting the vessel walls based on B-mode and/or color flow data, estimating the vessel orientation based on the best vessel edge data, and then computing the Doppler angle.
U.S. Pat. No. 4,937,797 describes a method of adjusting the transducer array beamforming delays to automatically steer the Doppler beam to achieve a target Doppler angle such as 60 degrees (or less). This method, however, requires the user to first manually rotate an angle cursor on the B-mode image to define the vessel orientation or flow direction.
There is a need for an automatic method of initializing and adjusting the Doppler sample gate position and size settings and the beam steering angle setting based on actual vessel image data, with the goal of improving the efficiency of the Doppler study above and beyond what can be achieved using presets.